The present invention generally relates to pointing devices, in particular for controlling the position of a cursor on a screen, such as the display of a personal computer, workstation or other computing devices having a graphic user interface. Such pointing devices may for instance include mice, trackballs and other computer peripherals for controlling the position of a cursor on a display screen.
The present invention more particularly relates to the field of optical pointing devices which comprise an optical sensing device including a photodetector array for measuring the varying intensity pattern of a portion of a surface which is illuminated with radiation and for extracting information about the relative motion between the photodetector array and the illuminated portion of the surface.
Optical pointing devices are already known in the art. U.S. Pat. No. 5,288,993, which is incorporated herein by reference, for instance discloses a cursor pointing device utilizing a photodetector array and an illuminated target ball having randomly distributed speckles. U.S. Pat. No. 5,703,356 (related to the above-mentioned U.S. Pat. No. 5,288,993), which is also incorporated herein by reference, further discloses (in reference to FIGS. 23A and 23B of this document) an optical cursor pointing device in the form of a mouse which does not require a ball and wherein light is reflected directly from the surface over which the pointing device is moved.
The imaging technique used in above-cited U.S. Pat. Nos. 5,288,993 and 5,703,356 in order to extract motion-related information is based on a so-called xe2x80x9cEdge Motion Detectionxe2x80x9d technique. This xe2x80x9cEdge Motion Detectionxe2x80x9d technique essentially consists in a determination of the movement of edges (i.e. a difference between the intensity of pairs of pixels) in the image detected by the photodetector array. Edges are defined as spatial intensity differences between two pixels of the photodetector array. The relative motion of each of these edges is tracked and measured so as to determine an overall displacement measurement which is representative of the relative movement between the photodetector array and the illuminated portion of the surface.
More particularly, according to U.S. Pat. No. 5,288,993, edges are determined between pairs of pixels aligned along a first axis of the photodetector array (for example in each row of the photodetector array) and between pairs of pixels aligned along a second axis of the photodetector array (for example in each column of the photodetector array). FIG. 10 depicts three pixels of the photodetector array, a first pixel or current pixel P, a second pixel Pright aligned with the first pixel P along a first axis 101, and a third pixel Pup aligned with the first pixel P along a second axis 102. Pixels Pright and Pup are show as being disposed on the right side and top side of pixel P for the purpose of explanation only. It will be appreciated that axes 101 and 102 may be orthogonal (as shown) or alternatively non orthogonal. It will also be appreciated that the pixel are not necessarily disposed so as to form a regular array having rows and columns. Other suitable arrangements may very well be envisaged.
For the purpose of simplification, the pixels of FIG. 10 are either shown as being white or black, a black pixel denoting an illuminated pixel. In this case, pixel P is illuminated and first and second edge conditions Ex, Ey exist respectively between pixels P and Pright along the first axis 101 and between pixels P and Pup along the second axis 102.
According to U.S. Pat. No. 5,288,993 and U.S. Pat. No. 5,703,356, the displacement measurement is evaluated, on the one hand, based on a normalized difference between the number of edges Ex which move in a first direction along the first axis 101 and edges Ex which move in the opposite direction along the first axis 101 (for example edges which from left to right and right to left in each row of the photodetector array), and, on the other hand, based on a normalized difference between the number of edges Ey which move in a first direction along the second axis 102 and edges Ey which move in the opposite direction along the second axis 102 (for example edges which move downwards and upwards in each column of the photodetector array).
Relative motion of edges is determined by comparing the position of these edges in the photodetector array at a first point in time with the position of edges in the photodetector array at a subsequent point in time. The optical pointing device thus typically comprises a light source (such as an infrared LED) which intermittently illuminates the portion of the surface in accordance with a determined sequence, and the pixel outputs of the photodetector array are sampled in accordance with the determined sequence to provide two successive sets of edge data that are compared to each other in order to determine a relative motion measurement.
According to one embodiment of U.S. Pat. No. 5,288,993 and U.S. Pat. No. 5,703,356 a differential technique is advantageously used in order to determine an edge condition between two pixels. According to this embodiment, an edge is defined as laying between two pixels if the ratio of intensities of the two photosensitive elements is larger than a determined level. An edge may thus be defined mathematically by the following Boolean expression:
Intensity[PIXEL 1] greater than K Intensity[PIXEL 2]
OR
K Intensity[PIXEL 1] less than Intensity[PIXEL 2]xe2x80x83xe2x80x83(1) 
where K is the selected scaling factor.
It will be appreciated that the first and second parts of the above expression, taken individually, each define an edge condition between the two pixels.
According to U.S. Pat. No. 5,703,356, the differences of intensities or edges between pixels is sensed as a difference in currents. More particularly, FIG. 17A of this document shows a differential current sensor for detecting an edge condition between two pixels. FIG. 1 of the present specification illustrates this differential current sensor. In this example, the current iout generated by the photosensitive element 1000 of the pixel is applied (after charge amplification by means of the charge amplifier 1705) on a input branch 1710A-B of a current mirror comprising eight output branches 1710, 1715, 1720, 1725, 1730, 1735, 1740 and 1745, four of which (output branches 1710, 1715, 1730 and 1735) output a non-scaled image of the input current iout. The other four output branches 1720, 1725, 1740, 1745 output a scaled image of the input current iout (K times the input current iout), the scaling factor K being defined by an adequate choice of the dimensions of the corresponding transistors of the current mirror. Two output branches 1715, 1720 supply the image io1I and the scaled image ioKI of the input current iout to the pixel on the left. Similarly two output branches 1735, 1740 supply the image io1d and the scaled image ioKd of the input current iout to the pixel below.
The differential current sensor further comprises two pairs of comparator circuits 1750A-1750B and 1750C-1750D, one pair 1750A-1750B for determining the edge condition, denoted Ex, between two pixels in the same row (in this case between the current pixel and the pixel on its right), the other pair 1750C-1750D for determining the edge condition, denoted Ey, between two pixels in the same column (in this case between the current pixel and the pixel on top). Each comparator circuit has one input connected to a non-scaled output 1710, 1730 or scaled output 1725, 1745 of the current mirror and a second input connected to a non-scaled output (supplying current ii1r, ii1u) or scaled output (supplying current iiKr, iiKu) of the current mirror of the pixel on the right or of the pixel on top. In this example, the outputs of each pair of comparator circuits are additionally combined by means of a logic NAND gates 1765, 1775 to provide a corresponding edge condition.
The output of one current comparator 1750A of the first pair is also combined by means of a logic NAND gate 1770 with the output of one current comparator 1750C of the second pair to provide an additional information about the intensity difference (denoted xe2x80x9cCOLORxe2x80x9d and referenced C) of the pixel as compared to the adjacent pixels, in this case as compared to the pixel on the right or the pixel on top.
According to U.S. Pat. No. 5,703,356, the two edge conditions Ex, Ey and the additional information C are thus defined mathematically by the following Boolean expressions:
Ex=Intensity[pixel] greater than K Intensity[pixel on right]
OR
K Intensity[pixel] less than Intensity[pixel on right]xe2x80x83xe2x80x83(2) 
Ey=Intensity[pixel] greater than K Intensity[pixel on top]
OR
K Intensity[pixel] less than Intensity[pixel on top]xe2x80x83xe2x80x83(3) 
C=Intensity[pixel] greater than K Intensity[pixel on right]
OR
Intensity[pixel] greater than K Intensity[pixel on top]xe2x80x83xe2x80x83(4) 
The outputs of the NAND gates 1765, 1770, 1775 are further connected to latch elements 1760A, 1760B and 1760C for at least temporarily storing the corresponding previous results, oEx, oEy and oC designating the previous outputs of the NAND gates 1765, 1770, 1775.
An alternative to the solution of FIG. 1 consists in providing, for each pixel, an integrating circuit, a scaling amplifier and an adequate number of comparator circuits. FIG. 2 schematically shows an example of such an alternative solution. In this example, the current iout generated by the photosensitive element 1000 of the pixel is applied on a input of an integrating circuit 1100 in order to generate an output voltage Vout. As illustrated in FIG. 3, the integrating circuit 1100 typically consists of an operational amplifier 1110 and a capacitive element 1120 having a determined capacitance C, the capacitive element 1120 being connected between the output and the inverting input of the amplifier 1110, the photosensitive element 1000 being connected to the inverting input of the amplifier and the non-inverting input of the amplifier being tied to a reference potential such as ground. The integrating circuit 1100 accordingly outputs a voltage signal Vout, or integrated signal, which varies over time and which is in essence the result of the integration over time of the current signal iout. Assuming that current iout has a substantially constant value during the period where integrating circuit is active (i.e. during a so-called integration period), the output voltage Vout will vary substantially linearly over time.
The voltage signal Vout is applied to a first input of two comparator circuits 1300A, 1300C and on the input of a scaling amplifier 1200. This scaling amplifier 1200 is designed to output a voltage signal which is a scaled image, denoted KVout, of signal Vout. The non-scaled voltage signals, denoted V1r and V1u, from the pixel on the right and the pixel on top, are respectively applied to a first input of two additional comparator circuits 1300B and 1300D. The scaled voltage signal KVout supplied by the scaling amplifier 1200 is applied on the second input of these two comparator circuits 1300B and 1300D. Similarly, the scaled voltage signals, denoted KVr and KVu, from the pixel on the right and the pixel on top, are respectively applied to a second input of comparator circuits 1300A and 1300C.
Similarly to the example of FIG. 1, NAND gates 1400A, 1400B and 1400C are provided to logically combine the outputs of the comparator circuits in order to generate the edge conditions Ex, Ey and the additional information C.
One serious disadvantage of the two examples illustrated in FIGS. 1 and 2 resides in the fact that they both require specific circuitry for generating the scaled image of the signal outputted by the photosensitive elements. This circuitry thus reduces the available die area and increases the power consumption and complexity of the sensing device.
An additional disadvantage of the above two examples resides in the fact that one has very little control on the scaling factor K of the circuit. Once this scaling factor is defined during fabrication by an adequate choice of the dimensions of the corresponding electronic components, this scaling factor cannot be adjusted by the user.
It addition, according to the prior art solution, the scaling factor K is typically adjusted so that the sensing device is less sensitive to analog measurement noise. In practice, it would be desirable to implement a hysteresis function in the sensing device. According to the prior art solution, one would again have little control and adjustment capability of this hysteresis function.
Accordingly, it is an object of the present invention to provide a solution that requires less die area, allows power consumption to be decreased, and the architecture of the sensing device to be simplified.
It is another object of the present invention to provide a solution that shows greater flexibility and in particular, that allows adjustment of the scaling factor K and/or easy implementation of a hysteresis function.
According to a first aspect of the invention, there is provided a method for comparing light intensity between pixels of a photodetector array, each of the pixels comprising a photosensitive element generating a sensed output signal in response to radiation, this method comprising the steps of:
integrating the sensed output signals over time to provide an integrated signal for each of the photosensitive elements;
interrupting the integration of a first sensed output signal of a first pixel at the end of a first time period and storing the resulting first integrated signal
continuing the integration of a second sensed output signal of a second pixel until the end of a second time period to provide a second integrated signal; and
comparing the first and second integrated signals to provide an output signal representative of an edge condition between the first and second pixels.
According to a second aspect of the invention, there is also provided a sensing device for an optical pointing device comprising a plurality of pixels including a first and a second pixel aligned along a first axis, each one of the pixels comprising:
a photosensitive element for generating a sensed output signal in response to radiation; and
an integrating circuit connected to the photosensitive element for integrating the sensed output signal over time and for outputting a resulting integrated signal,
the sensing device further comprising first comparator means for comparing light intensity between the first and second pixels and for determining if a first edge condition exists between the first and second pixels,
wherein the first comparator means comprise a first comparator circuit having one comparator input connected to the integrating circuit of the first pixel and another comparator input connected to the integrating circuit of the second pixel,
the sensing device further comprising:
means for resetting the integrating circuits during a resetting period and for releasing these integrating circuits during an integration period
means for disconnecting a first comparator input of the first comparator circuit from the corresponding integrating circuit at the end of a first time period;
means for storing the resulting integrated signal on the disconnected first comparator input of the first comparator circuit; and
means for latching the first comparator circuit at the end of the integration period.
According to a preferred aspect of the invention, there is provided a method and sensing device as defined above, wherein the integration period of the second pixel output signal has a first duration or a second duration shorter than the first duration depending on a previous state of the comparator circuit output. According to this preferred aspect of the invention, a hysteresis function is implemented. According to a specific embodiment, the second duration of the integration period may be selected to be equal to the duration of the first time period during which the first pixel output signal is integrated.
An optical pointing device including the above sensing device is also the object of the present invention.
According to the present invention, a time-based scaling scheme is implemented thereby allowing the scaling circuits of the prior art to be eliminated. As a consequence, die area as well as power consumption and complexity is reduced. In addition, the scaling factor may simply be adjusted by changing the ratio between the first time period and second time period (also referred to as integration period). Furthermore, this time-based scaling scheme may easily be adapted to implement a hysteresis function that allows sensitivity to noise to be reduced.
Level detection means may be provided for detecting when a first one of the integrated signals generated by the pixel integrating circuits reaches at least first and second determined levels. Accordingly, the first and second time periods are defined by the time for the first one of the integrated signals to reaches these first and second levels. In this case, this time-based scaling scheme also allows time-division to be used to separate analog and digital circuit operations, and in particular inhibit the clock signal supplied to processing means of the optical pointing device. This clock signal inhibition during analog measurement (i.e. during the integration period) eliminates digital impact (coupling, noise) to analog operations, thereby minimizing analog circuit errors and maximizing system sensitivity.
Last but not least, the above solution allows to maximise contrast over the area of the die since the integrating circuits will run until the integrated signal of the brightest pixel of the photodetector array reaches the maximum integration level. The integration time will therefore be dependent on the illumination level, and contrast, for a given illumination level, is always maximized.
Contrast ratio sensing may also easily be implemented by testing all pixels, upon the end of integration, via an analog circuit to detect the minimum integration level. This provides a measurement of the actual signal contrast seen by the sensor array.
Other aspects, features and advantages of the present invention will be apparent upon reading the following detailed description of non-limiting examples and embodiments made with reference to the accompanying drawings.